Effective immune responses require the development of T lymphocytes specific for antigens presented by self major histocompatibility complex (MHC) class I and class II molecules. Many basic and clinical studies require the detection and enumeration of antigen- specific T lymphocytes, but until recently their detection was inaccurate and cumbersome, requiring extensive tissue culture procedures. New reagents have been developed, comprised of four identical MHC molecules, each presenting a defined antigenic peptide. The reagent is fluorescently labeled to enable the direct detection of T cells that bind the reagent by their specific antigen receptor molecules. Previous research has established the approach for class I MHC molecules that bind to cytotoxic T cells, but reagents based upon class II MHC molecules, which would detect helper T lymphocytes, are less well-developed. This contract will focus on the development and utilization of class II MHC-peptide tetramers to detect and study antigen specific T cells. Studies will include: a) testing various forms of class II MHC-peptide complexes to optimize the reagents, b) demonstrate the reagents~ ability to detect human T cells in clinical samples and c) combine detection with functional T cell assays to increase the information obtained. Because helper T cells are central to virtually all immune responses, this technology is applicable to studies in many areas, including basic immune mechanisms, infectious diseases, vaccination, autoimmunity, transplant rejection, and tumor therapy.